Yawning&emdash;a deep inspiration with mouth
opening and slow expiration&emdash;is a
widespread behaviour amongst all vertebrates.
The behavioural repertoire of organisms
distantly related to each other such as fish,
birds and mammals includes yawning (Walusinski
and Deputte, 2004), although most of our
understanding about the prominent features of
yawning (e.g. contagion) came from studies in
humans (for an extensive review, see Provine,
2005). In spite of this ubiquity and the
relative ease with which yawning can be
identified and yawns counted, the biological
significance of yawning is unknown. The growing
number of functions linked to yawning reflects
the diversity of phenomena to which it has been
associated. All of these functions, however, may
be grouped as physiological (true or rest yawn)
or communicative (e.g. emotion or tension
yawn).

Most of the physiological hypotheses of
yawning's significance are based on a
restorative function (i.e. homeostatic
mechanism), including the opening of Eustachian
tubes (Laskiewicz, 1953), equilibrium of CO2
and/or O2 levels in the blood (Sauer and Sauer,
1967; Provine et al., 1987b), prevention of
atelectasis (Cahill, 1978), correction of
imbalance in cerebral oxidative metabolism
(Lehmann, 1979), proper articulation of the
temporomandibular joint (de Vries et al., 1982),
evacuation of potentially infectious substances
from the tonsils (McKenzie, 1994), brain
thermoregulation (Gallup, 2007), stimulation of
the carotid body by compression (Matikainen and
Elo, 2008), auto-regulation of the locomotor
system (Bertolucci, 2011), and more recently, a
process of switching the default mode network to
the attentional system through the capacity of
yawning to increase circulation of cerebrospinal
fluid (Walusinski, 2014). However, none of the
physiological hypotheses have received
sufficient empirical support.

Yawning behaviour has also been associated
with the presence of physiological disorders.
For example, yawning may indicate encephalitis
(Wilson, 1940), haemorrhage (Nash, 1942), motion
sickness (Graybiel and Knepton, 1976; Matsangas
and McCauley, 2014), the beginning of
hypoglycaemia, which is a prodromal sign of
vaso-vagal reaction (Cronin, 1988), and stress
(Kubota et al., 2014). It is still unknown
whether there is a cause-effect relationship
between yawning and these physiological
disorders, or if they are simply coincidental
manifestations of a regulatory function.

Yawning frequency has also been associated
with increased cholinergic and peptidergic
activity (Dourish and Cooper, 1990), and
decreased dopaminergic activity (Dourish and
Cooper, 1990). These associations may contribute
to the understanding of the immediate mechanisms
involved in yawning behaviour, but they hardly
say anything about the biological meaning of
yawning.

Most communicative hypotheses about yawning
come from observational studies performed in
non-human primates in whom teeth-bearing during
yawning has led to the suggestion that this
behaviour reinforces dominant identity rather
than signalling a threat (Deputte, 1994).
Accordingly, yawning has been compared with
intimidating displays that dominant males
usually show to subordinate males (Troisi et
al., 1990; Walusinski and Deputte, 2004; Zucker
et al., 1998). In contrast, Sauer and Sauer
(1967) proposed that yawning might induce
relaxation of social tension. Yawning has also
been described as a displacement
activity&emdash;an unexpected display by an
animal that appears to be engaged in other
activity (Delius, 1967; Troisi, 2002)&emdash;and
as an indication of changes in behavioural state
(Provine et al., 1987a; Greco et al., 1993).
There is also little experimental evidence to
support these hypotheses.

What makes the identification of yawning
with a function even more complicated is the
fact that the physiological correlates of
yawning, which might be used to support a given
hypothesis, may produce contrasting effects.
Thus, stimulation or inhibition of yawning has
frequently been associated with changes in
stress/anxiety, from the mild stress produced by
exploration (e.g. a rat placed in a new
environment) or vigilance, to the strong stress
produced by fear (e.g. response-dependent
punishment). For example, a new environment
inhibits a rat's yawning, as compared with the
increase observed following repetitive exposure
to the same observation setup (Moyaho and
Valencia, 2010). Low levels of vigilance have
been linked to yawning occurrence in young human
adults (Provine and Hamernik, 1986). Moreover, a
mild shock to the feet in rats failed to
suppress the yawning which preceded it (Moyaho
and Valencia, 2010). Although these results
clearly show a relationship between yawning and
stress/anxiety, they do not indicate a
consistent pattern; this may be partly due to
variables which cannot be quantitatively
controlled, making it difficult to determine
precise cause-effect relationships. The use of
quantitative variables such as defecation
rate&emdash;an indication of emotional reaction
to external stimuli which reflects the
physiological state of the body&emdash;may
clarify the relation of yawning with factors
that cause stress to an animal, and thus help to
recognise yawning functions.

Equating physiological and communicative
functions of yawning with non-social and social
contexts respectively is inaccurate since both
forms may occur in a social context. A more
precise identification would be with
non-directed yawning (i.e. performed without the
intention of eliciting a response from another
individual) and directed yawning (i.e. performed
with the intention of eliciting a response from
another individual) respectively (Hall and
Devore, 1965; Anderson, 2010). Nonetheless, it
is not yet clear how to distinguish them in a
social context. If a given animal yawns in front
of conspecifics, this might be considered as
directed yawning (i.e. display behaviour).
Conversely, if an animal yawns without being
watched by conspecifics, this could be taken as
an instance of non-directed yawning (i.e.
self-directed behaviour). Nevertheless, it would
be unjustified to assume that other sensory
modalities do not also participate in the
detection of yawning; therefore, apparently
non-directed yawning could indeed be directed.
Recently, Moyaho et al. (2015) scored yawning
rates in cage mate and stranger rats that were
exposed in pairs either to auditory, olfactory
or visual cues. The authors found that only
stranger rats showed auditory contagious
yawning, although cage mate rats showed
correlated defecation rates, a possible
indication of emotional empathy. These
differences between cage mate and stranger rats
confirm the participation of sensory cues other
than visual cues in yawning, and suggest that
the differential effect of the treatments
applied to the two groups of rats might help in
recognising yawning functions.

This study tested the hypothesis that the
differences which cage mate and stranger rats
showed in terms of yawning reflect two different
functions of yawning. For this purpose, the
study re-analysed the data obtained by Moyaho et
al. (2015) using probabilistic and statistical
models.

Discussion

The purpose of this study was to re-analyse
a database from a previous study, which
suggested that the differential response of cage
mate and stranger rats to a combination of
sensory cues could be used to identify the
function of yawning. With the re-analysis,
directed and non-directed yawning were
respectively identified with the response of
stranger and cage mate rats to the combination
of olfactory and auditory cues. The rats used
the olfactory cues to discriminate between cage
mate and stranger rats, and the auditory cues to
detect and respond to each other's yawning.

Yawning frequency could be a reliable
indication of a rat's physiological state, as
yawning rate and defecation rate (an index of
emotional reactivity) showed a consistent
negative association in cage mate rats, so that
frequent yawning was a genuine indication (i.e.
a cue) of a low-arousal state of calm. This
interpretation agrees with a steady increase in
yawning frequency recorded over several days in
HY male rats placed daily in the same
observation cages. This increase presumably
happened because the rats became acquainted with
the test condition (Moyaho and Valencia, 2010),
which might have progressively caused less
stress. While average yawning rate per test
situation decreased with average defecation rate
in cage mate rats&emdash;so that the greater the
uncertainty about the next cage rat's identity
the greater the defecation rate (i.e., NVOC
rats)&emdash;the rat of each pair that surpassed
the other in yawning frequency was the one that
tended to show the lower pre-test defecation
rate. Thus, somehow the relative yawning
frequency of each pair of cage mate rats
reflected a difference acquired before the test
situation to which they were exposed.

The existence of a negative association
between yawning and defecation rate in cage mate
rats contrasts with the lack of such an
association in stranger rats. Nonetheless, the
pairs of stranger rats with olfactory
communication yawned more frequently with higher
defecation rates, although the relation was
moderate. A thorough analysis of this relation
revealed the existence of a communicatory effect
within each pair of rats, since the yawning of
the rats with fewer yawns positively correlated
with the defecation rate of the rats with more
yawns. Similarly, the yawning of saline-treated
rats showed a positive association with the
defecation rate of the kanamycin-treated rat.
Therefore, stranger rats affected one another's
behaviour most likely through yawning.

The ways in which cage mate and stranger
rats used yawning behaviour in response to
olfactory and auditory cues are aligned with the
distinction between a cue and a signal (Maynard
Smith and Harper, 2003). A cue is a feature of
the world, animate or inanimate, that an animal
can use as a guide to future action (Hasson,
1994), and a signal is an act or structure that
an animal uses to change the behaviour of
another animal. The effect that the act
produces, fosters its evolution, and the
evolution of the receiver's response promotes
its effectiveness (Maynard Smith and Harper,
2003). In the case of cage mate rats with
olfactory communication, there must have been
quick individual recognition based on
familiarity and previously established dominance
hierarchies, which has been suggested to occur
in many animals (Bradbury and Vehrencamp, 1998).
Because there was no physical contact between
the rats, volatile chemicals likely played a
role in mutual recognition, and auditory signals
likely played a role in coordinating responses.
It is unlikely, however, that the yawning
observed in the rats exposed to olfactory
communication was a direct response to volatile
chemicals, although this could be the case in
male bats that yawned during social interactions
(Gebhard, 1997; as cited by Voigt-Heucke et al.,
2010). In any case, cage mate rats probably
recognized each other readily and adjusted their
behaviour accordingly; in this context of
familiarity, no conflict regarding
dominant-subordinate roles should exist between
the rats because a dominance hierarchy has
already been established. If so, cage mate rats
would mostly be irresponsive to each other's
behaviour, as evidenced by the finding that they
would choose to yawn in association with their
own defecation rate. Therefore, this type of
yawning can be referred to as a cue which may
have evolved into a regulatory act associated
with a rat's physiological state.

We specifically propose that cue yawning is
a motor act used to diminish variation in muscle
tone. This function is reasonable as a
hypothesis because of the type and large number
of muscles involved in yawning (approximately 54
in humans; Walusinski, 2004). These muscles
participate in proprioception and interoception
by conveying information used to indicate how an
individual feels (Craig, 2003; Walusinski,
2006). Moreover, it is known that the main
function of proprioceptive reflexes is to adjust
the motor output according to the biomechanical
state of the body and limbs; thereby a
compensating mechanism for the intrinsic
variability of such output is achieved (Pearson
and Gordon, 2000). Thus if yawning is an
involuntary motor act, it would be part of the
proprioceptive reflexes involved in decreasing
the intrinsic variability of muscle
tone&emdash;something like tuning up a musical
instrument&emdash;so as to ensure operation
efficiency.

The hypothesis proposed by Bertolucci
(2011)&emdash;that yawning (he did not
distinguish between cue yawning and signal
yawning) increases the level of tone necessary
for activity&emdash;explains the frequency of
yawning observed, for example, following
awakening, but not the frequency preceding sleep
onset, when it is known that most mammals also
yawn. The hypothesis that cue yawning reduces
the variation in muscle tone accounts for the
increase of yawning observed before a change of
state, either from resting to activity or from
activity to resting. Moreover, if the hypothesis
is correct, intra- and inter-individual
variation in yawning rates would reflect the
magnitude of muscle tone variation in each
individual, and thus the number of yawns
required to decrease it. Thus, the subjective
ratings of feeling associated with yawning can
vary from unpleasant to pleasant according to
the reduction of muscle tone variation achieved
by an individual. As a consequence of a pleasant
state following yawning, an individual may show
unconcern or indifference (Baenninger and Greco,
1991), and thus the possibility of agonistic
behaviour (e.g. dispute, conflict, etc.)
decreases.

In contrast to cage mate rats, an encounter
between stranger rats most likely initiates
competitive scent marking so that each rat can
determine the other's individual features as
well as its ability to defend territory and
resources (Hurst and Beynon, 2004). Scent
marking and counter-marking are advertising
strategies commonly used by rodents to establish
control over resources (Hurst and Beynon, 2004);
the chemicals released function as a reliable
mechanism by which rats can provide information
about their strain, sex, individual identity
(Brennan and Kendrick, 2006), and current
reproductive and health status (Hurst and
Beynon, 2004). This creates a context of
conflict in which most individuals would attempt
to settle any dispute involving territory and
resource defence without a fight by using
signals to persuade each other to flee (Bradbury
and Vehrencamp, 2011).

The findings of the present study agree with
the existence of conflict&emdash;and concomitant
attempts at resolution&emdash;between stranger
rats, because those with more yawns and under
olfactory (or auditory) communication yawned in
response to the yawning of those rats with fewer
yawns; the rats with more yawns probably played
a dominant role and were more motivated. In
fact, previous studies have revealed that a
context of frequent male-male encounters might
promote yawning (Moyaho et al., 2009). Moreover,
the rats with more yawns might also have high
levels of steroid hormones. There is a
significant positive association between yawning
and penile erections (Holmgren et al., 1985;
Moyaho et al., 2015) that depends on the action
of steroid hormones (Melis et al., 1994; Phoenix
and Chambers, 1982), which also facilitate
aggressive behaviour. Therefore, the rat with
more yawns of a pair might also have more
testosterone, and hence better odds of winning
an eventual fight. Thus, yawning frequency might
be an honest signal of physiological capacity in
stranger rats (Moyaho et al., 2015).
Nevertheless, for yawning to be an honest
signal, it should entail a cost. Although no
studies have assessed the costs of yawning, its
motor complexity suggests that it could be
physiologically exhausting&emdash;a large number
of muscles are recruited&emdash;and also a risky
act, because whenever an individual yawns it
becomes vulnerable to predation, as it
exaggeratedly opens its mouth and closes its
eyes. In addition to these potential costs,
there is evidence that males have low survival
rates caused by testosterone-dependent
behavioural traits which are necessary to
achieve a dominant status (Sinervo et al.,
2000).

Yawning behaviour could have evolved through
the ritualization of a cue resulting from
changes in physiological state (Maynard Smith
and Harper, 2003). Such a cue might be linked to
vomeronasal olfaction, which is involved in
intra-specific chemical communication (Bradbury
and Vehrencamp, 1998) in many mammals and
reptiles. For example, when mammals contact
urine or secretions, many of them (e.g.
antelopes, felines) raise their heads and
retract the upper lip to facilitate perception
of odorants. This is a behavioural pattern
called flehmen (Bradbury and Vehrencamp, 1998).
Indeed, there is a type of yawning in
chimpanzees in which the lips are funnelled
outwards that resembles flehmen (Vick and
Paukner, 2009). Similarly, a pumping mechanism
in hamsters facilitates perception of volatile
chemicals from the vomeronasal organ (Meredith
et al., 1980). Also, the occurrence of mouth
gaping in rattlesnakes, which is an analogue
behaviour to flehmen or yawning in mammals,
increases when snakes are exposed to conspecific
skin chemicals (Graves and Duvall, 1983), once
again apparently to facilitate vomeronasal
olfaction.

This type of chemical communication is
frequently associated with reproduction in both
mammals and reptiles (Meredith and
Fernandez-Fewell, 1994). In fact, the
vomeronasal organ is larger in males than in
females (Halpern, 1987), a size difference that
parallels the sexual dimorphism in yawning
behaviour, which in several species is more
frequent in males than females (Bertrand, 1969;
Deputte, 1994; Goy and Resko, 1972; Hadidian,
1980; Hall and Devore, 1965; Redican, 1975). It
is unlikely, however, that yawning behaviour is
currently used to stimulate vomeronasal
olfaction, given that some species which yawn
have lost the vomeronasal organ (e.g. fish and
birds; Bertmar, 1981), and given the
exaggeration of mouth opening. Nonetheless, the
ancestral origin of yawning and its broad
presence in animals are aligned with the
evolution of the vomeronasal organ, including
the lachrymal ducts (Bertmar, 1981), the content
of which is frequently released with yawning.
This coincidence, as well as the association
with spontaneous penile erections strongly
suggests that yawning behaviour arose from the
ritualization of pre-existing cues involved in
perceiving stimulating chemosignals in a mating
context.

Conclusion

To date, there is no convincing explanation
for the biological significance of yawning,
which remains an elusive issue. The variety of
contexts associated with yawning does not
necessarily mean yawning has several functions.
Instead, current literature suggests that
mammals, at least, tend to yawn in two major
situations: when an individual most likely
directs its yawning to a conspecific, and when
an individual does not direct its yawning to any
conspecific, because it is either alone or not
in the line of sight of the conspecific. In
effect, as the analysis of data presented here
shows, there seem to be two types of yawning;
the one shown by cage mate rats is an act
reflecting the physiological state of the body,
and therefore might be considered as a cue. We
propose that an individual yawns because it
needs to adjust muscle tone (i.e. decrease the
variation), so that the muscles can
appropriately work together when a change of
state is needed. The other type of yawning,
which stranger rats showed, involves the
interaction of at least two individuals in which
the yawning of one of them affects or is
affected by the other individual's behaviour.
Further studies are necessary to clarify the
precise direction of the effect; nonetheless, we
propose that this type of yawning might function
as a signal in male-male conflicts and probably
involves the physiological capacity of the
contestants. The consistent association of
yawning with penile erections and testosterone
seems to support this hypothesis.

In summary, the findings of this study
provide a framework in which earlier
categorizations for the function of yawning
converge into two expressions: cue yawning,
which may function as a regulatory act of the
level of muscle tone variation; and signal
yawning, which may function as a physiological
capacity signal.